Small Animal Cancer Imaqina Core Small laboratory animal models such as mice, rats and hamsters are widely used throughout the cancer research community at Washington University. Indeed, with the recent revolution in molecular biology, transgenic laboratory animal models, in particular, mice have become an indispensable part of the cancer research armamentarium. Animal models of cancer, however, can present the researcher with significant challenges in deciding how best to evaluate or analyze for the characteristics or effects of interest. Most often, for example, one wishes to follow each of the individual subjects that make up a sample population over an extended time period during which various procedures are carried out. Thus, invasive and/or destructive procedures, especially those that require sacrifice of the subject, are prohibitive. Under these circumstances, nondestructive imaging modalities, such as magnetic resonance imaging, positron emission tomography, and optical imaging are especially valuable. The goals and specific aims of the Small Animal Cancer Imaging Core remain essentially as initially described, namely to bring to the Washington University and St. Louis region cancer research communities a resource that offers state-of-the-art small animal magnetic resonance imaging, positron emission tomography, and optical imaging. As described in this document, optical imaging is being added as a new modality to the Core. The attributes of optical imaging complement those of positron emission tomography and magnetic resonance imaging. In particular, optical imaging offers extremely high signal detection sensitivity with target-specific (molecular) labeling while avoiding the difficulties inherent with radionuciides. Optical Imaging faces challenges in probing deep lying structures, a strength of PET, and in achieving high spatial resolution, a strength of MRI. Thus, the three modalities are complementary, rather than redundant, and offer great opportunities for multi-modal assessment of small animal cancer models.